Tuning circuits, such as super-heterodyne tuners are commonly used in communications equipment, such as radios, televisions and the like. A super-heterodyne tuner typically includes one or more mixers, used to frequency shift (or mix) a channel to be received to an intermediate frequency. At the intermediate frequency, the channel may be decoded/demodulated into a baseband signal.
The amount that a signal is shifted by the local oscillator depends on whether its frequency is higher or lower than the frequency of the channel to be tuned. For any mixing frequency, there are potentially multiple signals that could be shifted to the same intermediate frequency and interfere: one at the mixing frequency plus the intermediate frequency, another at the mixing frequency minus the intermediate frequency; and others at the intermediate frequency, plus or minus integer multiples of the mixing frequency.
Typically, only one of these is desired to be frequency shifted to the intermediate frequency. The remaining, undesired signals that may be inadvertently frequency shifted to the intermediate frequency are commonly referred to as image signals.
Several different techniques are known to eliminate image signals, prior to mixing. For example, image signals may be filtered using a variable frequency low pass, or band pass filter. Image reject mixers and/or complex mixers that add additional signals to remove the image frequency from the output may also be used. Likewise, some combination of filtering and image reject mixer circuitry may be used to ensure adequate rejection of signals at image frequencies.
All these techniques require relatively complex filter/mixing circuits that have their own shortcomings.
Accordingly, new image rejection methods and super-heterodyne receivers employing such rejection are desirable.